Reticles or masks with patterned images are essential components used in photolithography tools. Conventional tools use transmissive reticles. Specifically, for a transmissive reticle, a light source directs light through the reticle to project a patterned image on a semiconductor substrate covered with light-sensitive photoresist. The captured image in the photoresist defines integrated circuits on the substrate. Typically, tools using transmissive reticles can project features having line widths of 150 to 200 nanometer (nm) that form integrated circuits on the substrate.
In a conventional tool, when a substrate or wafer is ready for receiving a patterned image, the reticle is mounted on a reticle stage that supports and moves the reticle with a certain degree of freedom in the x and y directions. To protect the reticle from contamination, a pellicle can be used, which is a protective membrane that seals off the reticle. The pellicle, however, is not designed to prevent components from colliding with the reticle stage and damaging the reticle.
As integrated circuits become even more compact, the features of a patterned image projected on a substrate require smaller and finer detail. One photolithography technique capable of projecting such features with line widths of 30 nm or less is extreme ultraviolet (EUV) lithography. Instead of using a transmissive reticle, EUV lithography uses a reflective reticle in a vacuum chamber that reflects light with a wavelength in the range of 10 to 14 nm from the reticle to a substrate. A vacuum chamber is needed for EUV lithography to prevent EUV beam attenuation. EUV lithography also requires a number of mirrors and optical components that direct reflected light to the substrate. Such components can form an optical train or stage located below the reticle stage.
For EUV lithography, the reticle stage may require moving the reticle in close proximity to the optical stage. As a result, the reticle stage may move in the z direction as well as in the x and y directions for optical reasons. Consequently, safety features are necessary to prevent components in the optical stage from colliding with the reticle stage and causing irreparable damage to the reticle.
Since reticles are extremely expensive, there is a need to provide safety measures that protect the reticle stage and reticle from colliding with other components.